High voltage high current x-ray tube



Feb. 23, 1943. M! SLACK HIGH VOLTAGE HIGH CURRENT X -RAY TUBE Filed May 25, 1940 2 Sheets-Sheet l v 0 a H W J i n 1 z n M 8 u F jw w m INVENTOR Y B 5 5 a j v z t L M m r M 5 O 4 J 7 2 r fl a a ATTORNEY M 3? F@63T 30 WT WT 1943' T c. M. SLACK 2,311,705

HIGH VOLTAGE HIGH CURRENT X-RAY TUBE Filed May 25, 1940 2 Sheets-Sheet? ilii A'IV'TORNEY Patented Feb. 23, 1943 HIGH VOLTAGE HIGH CURRENT X-RAY TUBE Charles M. Slack, Glen Ridge, N. J., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 25, 1940, Serial No. 337,152

11 Claims.

The present invention relates to X-ray tubes and has particular reference to an X-ray tube capable of passing exceptionally high current.

X-ray tubes as now employed in the art are limited in electron current by the emission of the thermionic filamentary tungsten cathode, and in actual practice it has not been feasible for the current to exceed much more than several amperes. Recent workers have, however, constructed tubes capable of passing currents of several thousand amperes by using mercury pool cathodes and utilizing the mercury vapor in the tube to lower the space charge. These tubes are very erratic in operation and it is essential that the temperature of the mercury be kept below C., otherwise the discharge degenerates into a low voltage are producing no X-rays. Moreover, because of the mercury pool these tubes can be operated in only one position,

It is accordingly the primary object of the present invention to provide an evacuated X-ray tube which is capable of passing current of several thousand amperes. I

Another object of the present invention is the provision of an evacuated X-ray tube capable of passing current of several hundreds of amperes and which is exceptionally constant in operation.

Another object of the present invention is the provision of an evacuated X-ray tube capable of passing current of several hundreds of amperes wherein a non-thermionic electrode is employed.

Another object of the present invention is the provision of an evacuated X-ray tube capable of passing currents of several hundreds of amperes wherein electron emission is produced from a cold metal by a high potential gradient.

Another object of the present invention is the provision of an evacuated X-ray tube capable of passing current of several hundreds of amperes and an energizing circuit therefor whereby energization of the tube can be accurately controlled.

Another object of the present invention is the provision of an evacuated X-ray tube capable of passing current of several hundreds of amperes which simulates conventional type X-ray tubes in size and shape and is operable in any position.

Still further objects of the present invention will become obvious to those skilled in the art by reference to the accompanying drawings wherein:

Fig. 1 is a side view of an evacuated X-ray tube constructed in accordance with the present invention;

Fig. 2 is a fragmentary view of a modification which the evacuated X-ray tube of the present invention may take;

Fig. 3 is a fragmentary sectional view on an enlarged scale of the X-ray tube as shown in Fig. 1;

Fig. 4 is a sectional view taken on the line IV-'IV of Fig. l and looking in the direction indicated by the arrows;

Fig. 5 is a sectional view taken on ,the line V=V of Fig. 3 and looking in thedirection indicated by the arrows; and

Fig. 6 is a schematic diagram of an electrical circuit for energizing the X-ray tube.

Referring now to the drawings in detail, there is shown in Fig. 1 an envelope 5 having a reentrant portion 6 which is surrounded by a clamp l for supporting a refractory metal anode or target 8 such as tungsten and a leading-in wire 9 extends from the anode to the exterior of the envelope, being sealed thereto at H). To provide a flow of electrons, a ffield emissive cathode is shown which may comprise a sharp pointed rod l2, such as shown in Fig. 2, or a flattened. rod l3, as in Fig. 4 and of a material of low work function. Although a cathode of sharpened contour is herein shown, it is to be understood that other types of cathodes, such as rounded surfaces or even thermionic cathodes, may be utilized as hereinafter explained.

It has been long recognized in the art, ever since the advent of thermionic cathode X-ray tubes, that one of the most serious problems in connection with tubes of this type is that of auto=electronic emission. Inasmuch as this type tube is inherently self-rectifying, it is necessary to eliminate auto electronic emission from the various metallic parts and to completely degasify the tube to prevent the passage of an inverse current during the instant when the anode functions as a cathode. To this end the parts are given a rounded contour devoid of all points and edges to prevent high potential gradients which, at the operating voltages of the tube, would otherwise result in auto-electronic or field emission from the metallic parts.

Contrary to the above concept, I have provided an evacuated X-ray' tube wherein high potential gradients are employed to produce field emission of electrons which bombard the anode to generate X-rays with attendant flow of currents of several hundreds or even thousands of amperes. For example, the cathode I2 (Fig. 2) is spaced in close proximity to the anode 8 so as to produce a high potential gradientor field concentration at the cathode. Thus, when a potential of about 40 kilovolts or higher is impressed between the anode 8 and cathode I2, there will be a copious flow of electrons from the cathode to the anode due to the high gradient, with the electrons bombarding the anode to produce X-rays in the customary manner from the target surface, but with the added advantage that the current flow is exceptionally high. The spacing between the anode and cathode is critical for if too small, the electron discharge appears to degenerate apparently due to evolved metallic particles. Consequently, the spacing must be such that any vaporized tungsten does not have sufiicient time to travel to the cathode during the period of energization of the X-ray tube. On the other hand, if the spacing is too great, the electron discharge becomes uncertain unless extremely into a low voltage metallic arc.

electrons emitted by the cathode I 3. After assembly of the completed tube, it is exhausted through an exhaust tip 29.

By reference now more particularly to Fig. 6, an energizing circuit for the tube 5 is shown. This circuit comprises a high tension transformer provided with a primary winding 32 and a high voltage secondary winding 33 which generates a voltage of 40 k. v. or higher. The center point of the secondary winding 33 is grounded at 34 and one end. of this winding is connected through a rectifying valve tube to one side of a bank of condensers 36 and one terminal of a spark gap 31.

"r-In a similar manner the opposite end of the high voltages of the order of 100 kilovolts or higher are employed. Moreover, it is essential that, regardless of the type of cathode utilized, the electron flow must be due to field emission as hereinafter defined.

To eliminate this uncertainty without resorting to extremely high voltages, the preferred form of my invention, as shown in Fig. 1, incorporates an auxiliary firing electrode shown generally at M. By reference more particularly to Fig. 3 it will be seen that the envelope 5 is provided with a reentrant portion l5 to which a leading-inconductor I6 is sealed, as at H. Clamped about this reentrant portion [5 is the auxiliary firing anode l4 which, as shown, is in the form of a metallic cylinder connected to the leading-in conductor [6 by a wire or the like [8, and provided with openings I9 for exhaust purposes.

The end of the cylinder M has secured thereto, such as by screws or the like 20, an annular-metallic block 22 having an internal diameter but slightly less than that of the cylinder 14, with the open end of the block adjacent the anode converging to form parallel walls 23 extending normal to the longitudinal axis of the tubegas can be appreciated from Figs. 3 and 4. Thisblock 22 is provided with an integral partition 24 which thus results in the block forming a closure mem her for the cylinder l4 except for a slot 25 ex-' tending parallel to the walls 23, as shown in Fig.4.

The flattened end or beaver-tail cathode. 13. which may be formedon the end of a rod of thorium, molybdenum or the like, is positioned in the slot 25 and approximately on the longitudinal axis of the tube with the remainder of the rod insulated by a vitreous sheathing or the like 26, which passes through an opening 21 in the block 22, being sealed to a side arm 28 of the envelope 5. By reference more particularly to Figs. 4 and 5, it will be noted that the flattened or beaver tail cathode I3 thus naturally presents a very narrow edge transverse to the longitudinal axis of the tube and adjacent the nearest surface of the auxiliary electrode [4 which provides a surface having awidth or thickness of the same order of magnitude as, or at least no greater than, the spacing between the cathode and nearest surface formed by the slot 25 of the auxiliary electrode. Moreover, this edge of the cathode is disposed in very close proximity to the nearest point of the transverse portion of the auxiliary electrode to the anode for a purpose hereinafter secondary winding 33 is connected through a rectifying valve tube 38 to one side of a second bank of condensers 39 and also to a conductor 40 extending to the cathode l3 of the X-ray tube 5. The opposite side of the bank of condensers 36 is connected through the secondary winding 42 of a transformer 43 to ground as at 44, and the remaining side of the bank of condensers 39 is likewise connected to ground as shown at 45. A spark gap 46 is interposed between the respective banks of condensers 36 and 39 for a purpose hereinafter set forth.

The remaining terminal of the spark gap 31 is connected to the anode 3 of the X-ray tube and through a current limiting resistance 41 to the auxiliary firing anode l4. And in a similar manner a current limiting resistance 48 connects the cathode i3 to the anode 8. Upon energization of the primary winding 32 of the high voltage transformer 30, the bank of condensers 36 will be charged, during one half wave of the alternatin current cycle, by a circuit which extends from one end of the secondary winding, through the valve tube 35 to the condensers 36, and then from the condensers through the secondary winding 42 of the transformer 43 and by means of ground 44 to the center point 34 of the secondary winding 33.

. Likewise, during the same half wave of the alternating current cycle, the bank of condensers 39 is charged by a circuit extending from-the opposite end of the secondary winding 33, through rectifyingvalve tube 38 to the condensers 39, and thence through ground 45 back to the center point 34 of the secondary winding 33. When the bank of condensers 36 and 39 become charged to the breakdown voltage for which the gap 31 is set, they will discharge and energize the X-ray tube.

At the beginning of the discharge the potential of the auxiliary firing anode I 4 is initially the same as that of the main anode 8 and the total voltage available from the condensers is impressed across the main anode 8 and cathode I3, as well as across the auxiliary firing anode l4 and cathode l3.

Since the spacing between the sharp contoured cathode l3 and the surrounding firing anode i4 is short, a high potential gradient is present which draws electrons from the metal of the cathode due to the field concentration and hence may be termed field emission of electrons, as distinguished from thermionic emission from a heated electrode or emission due to positive ion bombardment, as occurs in the case of the gaseous type X-ray tubes.

The resulting phenomenon is of such short duration, the entire discharge consuming only approximately 2 micro-seconds, that it has been impossible up to the present time to determine exactly what occurs. My present theory evolved from observation of the tube is that the "field emission apparently causes hot spots on the cathode I3, resulting in a low voltage metallic arc of extremely short duration. Moreover, since the cathode l3 adjacent the nearest surface of the closely spaced auxiliary electrode is a relatively sharp edge and the high potential gradient is exceedingly high, metallic particles are apparently pulled out of a point on the cathode causing the formation of this low voltage metallic are accompanied by the generation of positive ions with attendant reduction of the space charge. In addition, since the shortest spacing between the auxiliary electrode and cathode is situated in close proximity to the nearest point of the trans verse portion of the auxiliary electrode to the anode, and the gap between the cathode and auxiliary electrode, over which the discharge is initiated, is at least as great as its width measured in the direction of the main discharge, the high electrostatic field readily influences the metallic arc discharge because it is in close proximity to the surface of the auxiliary electrode nearest the anode and not so far below this point that the electrostatic field has little or no influence. Accordingly, this high electrostatic field, together with reduction of the space charge due to the generation of positive metallic ions, causes an almost instantaneous copious flow of electrons from the cathode l3 to the anode 8 which enables the entire available energy of a magnitude of many hundreds or several thousand amperes to flow between the cathode and anode with bombardment of the latter with the production of X-rays. Whatever, the phenomenon, immediately following the discharge between the auxiliary firing anode l4 and the cathode l3 the current attempts to rise, but because of the resistance 41, the voltage drops and the potential of the auxiliary anode becomes approximately that of the cathode.

Consequently, the total discharge is transferred to the main anode 8 and the whole eifect occurs so rapidly that practically the total energy stored in the condensers 36 and 39 is dissipated in the electron bombardment of the anode 8 with ensuing generation of X-rays. Since the resistance 48 is in parallel with the discharge, the current flowing through the resistance is but a minute fraction of the several hundreds or more amperes flowing between the anode 8 and cathode l3, and such resistance serves to control the breakdown voltage of the gap 31 so that the same is always uniform for its particular setting.

In the system as thus far described, and assuming that the spark gap 46 is omitted and the condensers connected together in lieu thereof, the X-ray tube will be energized only when the condensers are charged to the breakdown voltage of the gap 31 or its spacing is decreased. In numerous experiments I have taken radiographs of many rapidly moving objects, such as bullets, by passing them through the gap 31. However, such method of control of the X-ray tube is disadvantageous from the standpoint of accuracy, particularly in commercial practice, because of the high voltage of the gap.

Accordingly, I provide a control system such as shown in Fig. 6 which comprises the transformer 43, the primary winding 52 of which is connected to ground at 44 and. to the anode 53 of a three-electrode discharge tube 54. The thermionic cathode 55 of this tube is connected to a source of energy, such as a battery 56, and receives heating current from a low voltage transformer 51, the primary winding of which of volts. The grid 58 of the tube 54 is also connected to the negative side of the battery 56 through a normally closed switch 59 and aleakage resistance 60 is connected in parallel with the cathode-grid circuit across the battery 56.

With the switch 59 in its normally closed position, a negative bias is accordingly impressed on the grid 58 which prevents flow of current in the plate circuit of the tube 54. This latter circuit includes, in addition to the primary winding 52, a condenser 62 which, as shown, is connected'to the cathode 55 and to ground as at 63. In order to charge the condenser 62 a transformer 64 is provided, the primary winding 65 of which may be also connected to the same source as that of the primary windings of the transformers 30 and 51.

The secondary winding 66 of this transformer 64, which generates a voltage of approximately 2000 volts, has one of its ends connected to ground at 61 and the other end thereof is connected to the thermionic cathode 68 of a rectifying valve tube 69, the plate electrode 10 of which is connected to the condenser 62. A second low voltage transformer 12 similar to the transformer-51 is provided for heating the thermionic cathode 68.

Upon closure of a switch (not shown), the transformer 64 is accordingly energized and charges the condenser 62 with unidirectional current through the rectifier 66, the charging circuit being completed through ground from 63 to 61. Discharge of the condenser does not occur because current flow in the discharge circuit is prohibited by the negative bias on the grid58 of the control tube 54, from the battery 56.

A suitable switch (not shown) may be closed to connect the primary winding 32 of the high tension transformer 30 to the customary source of domestic potential with the result that the banks of condensers 36 and 39 will be charged, in the manner previously described, to a voltage just slightly below the breakdown voltage of the gap 31. The spark gap 46 enables the low voltage side of the bank of condensers 36 to be raised above ground potential sufficiently to break down the main gap 31 and as the same time protects the secondary windin 42 from the high current flowing during the discharge.

When it is desired to energize the X-ray tube, the switch 59 is opened, thus removing the negative' bias from the grid 58 of the tube 54', which immediately leaks off through the resistance 60. The condenser 62 accordingly discharges through tube 54, energizing the primary winding 52 of the transformer 43. A high voltage impulse is thus generated in the secondary winding 42 which raises the charge in the bank of condensers 36, simultaneously breaking down the spark gaps 46 and 31 with attendant energization of the X-ray tube 5 in the manner previously described.

It thus becomes obvious to those skilled in the art that an evacuated X-ray tube is herein provided which employs a field emissive cathode and is capable of passing currents of several hundreds or more amperes. Moreover, due to the imposition of a high potential gradient at the cathode, field emission of electrons results with the electrons bombarding the anode to produce X rays. Also, a control circuit for the tube is herein provided enabling accurate energization of the X-ray tube at the desired moment.

Although two modifications of the present invention have been herein shown and described, it is to be understood that other embodiments thereof may be made without departing from the spirit may be connected to the usual commercial source 7 and scope of the appended claims.

I claim:

1. An X-ray tube comprising an envelope provided with an anode and a cathode of sharpened contour therein, and an auxiliary electrode spaced in close proximity to said cathode, said cathode having a thickness adjacent the nearest surface of said auxiliary electrode at least no greater than the spacing between said cathode and said auxiliary electrode, said auxiliary electrode being operable to produce a high potential gradient at the cathode and a metallic arc discharge therebetween with attendant field emission of electrons from said cathode which bombard said anode to cause the generation of X-rays upon the application of a high potential between said electrodes.

2. An X-ray tube comprising an enxelope provided with an anode and a cathode of sharpened contour therein, an auxiliary electrode spaced in close proximity to said cathode, means for initially applying substantially the same potential between said anode and cathodeand between said auxiliary electrode and cathode to produce a high potential gradient at said cathode with attendant field emission ofelectrons from said cathode to cause a momentary flow thereof from said cathode to said auxiliary electrode, and means operable to cause said auxiliary electrode to assume substantially the same potential as said cathode after the lapse of a minute period of time to cause the electrons from said cathod to bombard said anode with attendant generation of X-rays.

3. An X-ray tube comprising an envelope provided with an anode and a cathode of sharpened contour therein, and an auxiliary electrode spaced in close proximity to said cathode, said cathode having a thickness adjacent the nearest surface of said auxiliary electrode of the same order of magnitude as the spacing between said cathode and said auxiliary electrode, said auxiliary electrode being operable to produce a high potential gradient at said cathode and the formation of a metallic arc discharge accompanied by the generation of positive metallic ions which reduce the space charge and cause a copious flow of electrons from said cathode to said anode with attendant generation of X-rays and accompanied by high current flow upon the application of a high povtential between said electrodes.

4. AnX-ray tube comprising an envelope provided with an anode and a cathode of sharpened contour therein, and an auxiliary electrode in closer proximity to said cathode than to said anode and operable to cause field emission of electrons and a metallic arc discharge between said cathode and auxiliary electrode with attendant generation of positive metallic ions and reduction in space charge, said cathode having a surface whose smallest dimension and from which said metallic arc discharge originates adjacent the nearest surface of said auxiliary electrode at least no greater than the spacing between the closest surface of said cathode and auxiliary electrode, and said cathode surface from which the arc originates being in close proximity to the transverse surface of said auxiliary electrode nearest said anode so that the electrostatic field at said cathode is sufficient to cause a copious flow of electrons from said cathode which bombard said anode to produce X-rays accompanied by a high current fiow following the application of a high potential to said electrodes.

5. The combination with an X-ray tube provided with an anode and a cathode of sharpened contour and spaced in close proximity to said anode to produce a-high potentialgradient at said cathode with field emission of electrons from said cathode and attendant bombardment of said anode to produce X-rays, of an energizing system therefor comprising energy storage means, a source of electrical energy for accumulating a high potential charge in said energy storage means, and means interposed between said energy storage means and said X-ray tube and automatically operable upon the accumulation of a predetermined charge in said energy storage means to cause energization of said X-ray tube by said energy storage means.

6. The combination with an X-ray tube provided with a main anode and a cathode of sharpened contour and having an auxiliary anode in close proximity to said cathode to produce a high potential gradient with field emission of electrons from said cathode and attendant bombardment of said main anodeto produce X-rays, of an energizing system therefor comprising energy storage means, a source of electrical energy for accumulating a high potential charge in said energy storage means, means interposed between said energy storage means and said X-ray tube and operable to cause energization of said X-ray tube when said energy storage means becomes charged to a predetermined value, and an impedance device connecting said main anode to said auxiliary anode to cause the latter to assume substantially the same potential as said cathode after the lapse of a minute period of time following operation of said last mentioned means with attendant energization of said X-ray tube.

'7. The combination with an X-ray tube provided with an anode and a cathode of sharpened contour and having an auxiliary electrode in close proximity to said cathode to produce a high potential gradient at said cathode with field emission of electrons froni'said cathode and attendant bombardment of said anode to produce X-rays, of an energizing system therefor comprising energy storage means, a source of electrical energy for accumulating a high potential charge in said energy storage'means, means interposed between said energy storage means and said X-ray tube and automatically operable to cause energization of said X-ray tube by said energy storage means when the latter becomes charged to a predetermined value, and means operable at will to cause operation of said last mentioned means by suddenly charging said energy storage means to its predetermined value.

8. A high vacuum X-ray tube provided with an anode, a cold metallic cathode solid at room temperature, and an auxiliary electrode spaced in close proximityto said cathode to produce a high potential gradient at said cathode sufiicient to cause field emission of electrons with an attendant metallic arc discharge between said cathode and auxiliary electrode which generates positive metallic ions accompanied by reduction in space charge, and means for applying a high potential between said electrodes to cause a copious flow of electrons from said cathode which bombard said anode to produce X-rays accompanied by current flow between said cathode and anode having a magnitude of amperes.

9. A high vacuum X-ray tube provided with an anode, a cold metallic cathodesolid at room temperature, and an auxiliary electrode spaced in close proximity to said cathode to produce a high potential gradient at said cathode suflicient to cause field emission of electrons with an attendant metallic arc discharge which generates positive metallic ions accompanied by reduction in space charge, means for applying a high potential between said electrodes, and the spacing between said anode and cathode being great enough that copious electron flow with substantially no accompanying flow of evolved metallic particles occurs from said cathode which bombards said anode to produce X-rays accompanied by current flow having a magnitude of amperes.

10. A high vacuum X-ray tube provided with an anode, a cold metallic cathode solid at room temperature, and an auxiliary electrode spaced in close proximity to said cathode to produce a high potential gradient at said cathode sufficient to cause field emission of electrons with an attendant metallic arc discharge which generates positive metallic ions accompanied by reduction in space charge, means for applying a potential between said electrodes above approximately 40 kilovolts, and the spacing between said anode and cathode being great enough that copious electron flow without accompanying flow of evolved metallic particles occurs from said cathode which bombard said anode to produce X-rays accompanied by current flow having a magnitude of amperes.

11. A high vacuum X-ray tube provided with an anode, a cold metallic cathode solid at room temperature, and an auxiliary electrode, the spacing between said cathode and auxiliary electrode being so small that a high potential gradient is produced at said cathode sufiicient to cause field emission of electrons and the formation of a metallic arc discharge between said cathode and auxiliary electrode which generates positive metallic ions causing reduction of the space charge, means for momentarily applying a potential between said electrodes above approximately 40 kilovolts, and the spacing between said anode and cathode being great enough that copious electron flow without accompanying flow of evolved mei tallic particles occurs from said cathode .which bombard said anode to produce X-rays accompanied by current flow for a very few microseconds having a magnitude of amperes.

CHARLES M. SLACK. 

